(1) Field of the Invention
The invention relates to the general field of photoluminescent coatings, particularly as used in plasma displays.
(2) Description of the Prior Art
In plasma displays, most of the light that is generated by the gas discharge of the display is in the ultraviolet (UV) region. Some of this is converted to visible light through use of a photoluminiscent coating on the inside surface of the viewing screen. In the conventional arrangements associated with the prior art, much of the ultraviolet radiation that strikes the photoluminescent phosphor particles is reflected from their surfaces and is then lost to the display. Thus, the luminous efficiency of current plasma displays is typically between about 0.3 and 3.5%.
FIG. 1 illustrates this problem of undesired UV reflection. Phosphor particles, such as 1, form a layer on the inside surface of viewing screen 2. A ray of UV light 4 from gas plasma 3 strikes phosphor particle 5. Some of the UV is reflected as ray 6 while the rest penetrates the phosphor where photoluminescence occurs and visible light rays 7 are emitted. Clearly, if ways can be found to reduce the reflectivity of the phosphor particles at the wavelengths concerned, greater luminous efficiency of the display will result.
The present invention teaches use of a suitable dielectric coating on each of the phosphor particles as a way to reduce phosphor reflectivity in the UV. Although we have not found any prior art that describes this approach to the problem, there is mention in the prior art of coating phosphor particles:
Budd (U.S. Pat. No. 5,418,062 May 1995) coats electroluminescent phosphor particles with a view to protecting them against moisture during subsequent use. Possible effects on the reflectivity of the particles are not discussed since reflectivity of the particles is unimportant in electroluminescence. Budd uses Chemical Vapor Deposition (in a temperature range between 25 and 170.degree. C.) as the preferred method for coating the particles with an oxide layer between about 0.1 and 3 microns thick. There is no attempt made to control the thickness of the coating. In an attempt to coat the particles uniformly, particles are subjected to continuous agitation, using a fluidized bed, during CVD.
Okada et al. (U.S. Pat. No. 5,433,888 July 1995) coat phosphor particles primarily for adhesive purposes. As an additional benefit, the adhesive layer is a water soluble rare earth compound that leads to improved luminous flux conditions. Their coatings are formed by suspending their phosphor particles in an aqueous solution, applying this solution to a surface, and then baking to dryness.
Dutta et al. (U.S. Pat. No. 5,441,774 August 1995) use a liquid polymer as a vehicle for conveying phosphor particles onto a surface. The polymer is subsequently removed by heating it above its decomposition temperature. No coating remains on the phosphor particles but they end up with good adhesion to the surface.